In outboard engine units, various metal portions are exposed to seawater, and thus, metal members more prone to corrosion than the metal portions are sometimes attached to the metal portions with a view to preventing corrosion of the metal portions. The “metal members more prone to corrosion” are called “sacrificial electrodes” or “sacrificial anodes” because they are more easily ionizable and have a lower positive potential than the metal portions and dissolve due to corrosion in place of the metal portions. Examples of the conventionally-known mounting structures for such a sacrificial electrode include one where the sacrificial electrode is mounted to face a water jacket (see, for example, Japanese Patent Application Laid-Open Publication No. 2000-53086).
According to the disclosure of the 2000-53086 publication, an anode mounting port is formed in the water jacket provided in a cylinder block, and an anode functioning as a sacrificial electrode is fixedly fastened, via a bolt, to the inner surface of a lid member closing the anode mounting port. As the anode dissolves, the bolt fixedly fastening the anode to the lid member gradually gets loosened and may fall off from the cylinder block.
In order to prevent the aforementioned inconvenience, a sacrificial electrode mounting structure shown in FIG. 6 has been proposed, in which a groove portion 102 is formed in a cylinder block 101 and a lid member 103 is mounted on the cylinder block 101 to close the opening of the groove portion 102, and in which a water jacket 104 functioning as a cooling water passageway is defined by the groove portion 102 and the lid member 103. Further, a sacrificial electrode 107 is fixedly fastened to a wall 106 of the groove portion 102 by means of a screw 108, and the lid member 103 has a protruding portion 103a formed on the inner surface thereof in opposed relation to a head portion 108a of the screw 108. By the provision of the protruding portion 103a, the screw 108 can be prevented from getting loosened to fall off from the cylinder block 107. However, the protruding portion 103a that protrudes into the water jacket 104 would undesirably narrow the cooling water passageway and hinder a flow of the cooling water within the water jacket 104.